For your landscape you’ll need to make a parent material that will hold all your landscape textures you’re planning on using, inside this material you will be using a “landscape blend” which will be how you connect multiple textures to the material, you’ll need a landscape blend node for the main texture, normal maps, AO etc. Once you have your parent landscape material you can go into the landscape mode and change the material to your new parent material or if you already have the landscape you can click on the landscape in your world outliner and change the landscape material in the details panel. Before you can paint your materials you will need to go into landscape mode and open the Paint window, in there you will find your layers which will be reflective of what you’ve connected in your landscape blend node, you will need to create a no weight-blend by clicking on the dropdown box (you should place these in a new folder under your project in the content browser) and then you should be good to paint your textures.
Now as far as your sky goes that’s still something I’m personally looking into myself, you can add an “Environment Light Rig” device which is located in the Fortnite folder under the content browser then tick the box for “Disable All Time of Day Managers” in the world outliner panel which will remove the chapter 4 skybox but you will not have any clouds or a moon, I imagine you will need to create your own skybox if you’re trying to add stars and such it doesn’t seem like we have access to more skybox options at the moment unless I’m missing something.
Edit: Found some extra info on the sky settings, instead of using an “Environment Light Rig” device there is a “Day Sequence” device that will allow you to change up your skybox and play with cloud/sun/moon settings (you will need to make sure the “Disable All Time of Day Manager” box is unticked in the world settings to see this device in the island). That said still trying to figure out how you go about changing the skybox entirely.